Zero optical path difference is used to bring optical devices such as telescopes into phase. Optically phasing an array of multiple telescopes permits the formation of a synthetic aperture.
Synthetic apertures are formed when separate optical systems are combined to function as a single larger aperture. When an aperture is synthesized, independent optical systems are phased to form a common image field with resolution determined by the maximum dimension of the array and therefore exceeding that produced by any single element. By optically phasing an array of multiple telescopes, a synthetic aperture is formed which can achieve the performance of an equivalent sized, single large telescope.
Phased arrays are also modular. Ihey can be built in stages and to some extent, be operational as soon as the first telescope is operational. Such an array of independent telescopes has functional flexibility, several simultaneous operations can be carried out by individual telescopes within a synthetic aperture. For example, images can be directed to different cameras or spectrographic devices for simultaneous observation in separate imaging modes. When operated as a transmitter, a synthetic aperture has the option of sending beams in different directions.
Phased array apertures have virtually no size limitations. By modularly combining telescopes in a phased configuration, telescopes and transmitters of previously unimaginable sizes can be constructed. Large optics fabrication has historically posed an impermeable barrier to building large aperture telescope systems. By phasing a numter of reasonably-sized telescopes, this manufacturing barrier can be broken. Interferometers are devices that make use of the wave characteristics of light in order to measure length or changes in length with extreme accuracy. Michaelson interferometers are based on amplitude splitting of a light beam from a narrow band light source. Light from the light source is split into two beams by a 45.degree. beam splitting, partially reflective plate and is transmitted to two different reflecting surfaces. One of the reflecting surfaces is generally a reference mirror which reflects the split beam back to the beam splitting plate. A second reflective surface is formed at a telescope or other optical device for which optical path length difference is to be compared to the reference. The second reflective surface reflects light from the second of the split beams back to the beam splitter where the beams are recombined and directed to a receiver. The receiver may be a screen, photocell or human eye. Depending on the difference between the distances from the beam splitter to the two reflective surfaces the two beams will interfere constructively or destructively. This interference results in the formation of interference fringes at the receiver.
In white light, with its many wavelengths, the fringes can only be formed if the path difference in part of the field of view is made zero. This is because the spectral separation of the successive regions of constructive interference in white light are too close to be perceived. Since white light comprises different wavelengths (different colors of light) fringes appear colored close to zero optical path difference and disappear at larger path differences. If there is a one half cycle relative phase shift at the beam splitter, the fringe at zero optical path difference is black and can be distinguished from the neighboring fringes. Formation of the black fringe therefore discloses that the two surfaces (reference and telescope) are very close to zero optical path difference and nearly in phase.
White light fringes differ in total intensity very little or not at all from the very muted white light generally received. Further, light contrast between light and dark fringes (stripes) is generally very small. As a result, when using an interferometer to phase telescopes, simple photo optical systems for detecting hite light fringes have been unsuccessful. Manually observing the interferometer image to detect white light fringes is also a difficult and tedious job. The movable reflective surface must be very slowly moved over a period of hours for the fringes to be detected. Even at such slow scanning speeds, however, the low contrast fringes can easily be missed by a careful observer. Thus the phasing of multiple optical devices is a long tedious job.
In view of the above it is an object of this invention to provide an interferometer that is useful to rapidly determine zero optical path difference for the phasing of optical devices.
A further object of this invention is to provide a method of determining zero optical path difference using white light interferometry that is capable of overcoming variation in interference fringe contrasts.
Yet another object of this invention is to provide an automated system for the phasing of telescopes.